Many vibratory schemes exist which rely upon measuring a change in the mechanical resonant frequency of a system when a sample mass is introduced. But each has inherent practical and cost limitations.
To constrain linear motion along a single axis, some prior systems use air bearings that require compressor-regulator components and rather precise mechanisms, such as spring loaded sears to release the sample. Other systems incorporating a vibrating beam or other similar means require that the sample be placed in the same precise location in order not to change the moment arm and consequently the frequency characteristics of the system. Still other systems operate at very high mechanical frequencies requiring special substrates or mechanical means of attaching the sample to the pan as well as thermal isolation of the sample from heat generated by mechanical stresses attributable to these high operating frequencies. Others oscillate the pan in a plane perpendicular to the force of gravity and thereby can not benefit fully from the inherrent gravitational force present in terrestial applications, to maintain the sample on the pan without special substrates or mechanical coupling.